For electrical power converters operating at power levels larger than around 100 W at full load, a resonant topology is useful due to its high efficiency, small volume and high power density. At these power levels, the extra cost of resonant converters compared with other converters is compensated for by the additional advantages of a resonant topology. There are several types of resonant converters, using either half bridge or full bridge configurations, and the number of resonant components may vary.
A general circuit diagram of a series resonant converter 100 is illustrated in FIG. 1. The converter comprises a primary side circuit 101 and a secondary side circuit 102, with a transformer 103 common to the two circuits. The resonant components of the converter 100 comprise a capacitor Cr and inductor Ls on the primary side circuit 101, arranged in series and connected to a node 104 between a pair of switches 105a, 105b connected across a voltage supply 106. Operation of the pair of switches 105a, 105b is controlled by a switching controller 107 (not shown), which causes the switches 105a, 105b to open and close in a defined sequence and at a defined frequency. The timing of the switches can be varied to vary the output voltage Vout at the secondary side circuit 102. The output voltage may be regulated to a constant value, while delivering power to a desired load (known as Constant Voltage, or CV, mode), or the output may be regulated to deliver a desired current level when the load forces a certain output voltage (known as Constant Current, or CC, mode). As the magnetizing inductance of the transformer is relatively large (and ideally for a pure series resonant converter it is infinite) compared with the series inductor Ls, it effectively does not form part of the resonant circuit.
A low or zero voltage across each switch is desirable at the moment the switch is operated (i.e. closed, or made conductive), as this reduces switching losses and avoids damage to the semiconductor switches. This is generally known as soft switching. Because of the large value of the magnetizing inductance of the series resonant converter 100, the stored energy in the magnetizing inductance is not sufficient to provide for soft switching. The current in the series inductance Ls is therefore necessary to obtain soft switching.
Other types of converters derived from the basic series type resonant converter shown in FIG. 1 are also known, including multi resonant converters, in which more than two components take part in resonance. One such variant is the LLC converter 200, an example of which is illustrated in FIG. 2. Besides the series resonant components Ls and Cr, a magnetizing inductance Lm also takes part in the resonance. This configuration allows for operation at a frequency below series resonance in a so-called discontinuous mode, as the magnetizing inductance Lm allows for soft switching when the secondary side circuit diodes are not conducting.
Another variant is the LCC converter 300, an example of which is illustrated in FIG. 3, in which a second resonant capacitor Cp is provided in the secondary side circuit. An important difference compared with the LLC converter 200 is the difference in behaviour of the LCC converter 300 due to this parallel capacitor Cp, which results in a low output voltage at high switching frequencies. An LLC converter 200 gives a fixed output voltage at high switching frequencies.
LLC type converters are often operated with a 50% duty cycle at the half bridge node (i.e. the node between the pair of switches), with a variation of the switching frequency used to regulate the output power. This method gives an acceptable efficiency for medium to large loads. For low loads, however, there is a drawback of a relatively large circulating current. This results in a decreased efficiency at low load.